Owing to the limited fossil fuel resources and in particular to the limited deposits of mineral oil as raw material for the extraction of fuels for the operation of combustion engines, efforts are constantly being made in the development of internal combustion engines to minimize fuel consumption, the primary focus of these efforts being an improved, that is to say a more efficient combustion. On the other hand, however, specific strategies with regard to the basic operating principle of the internal combustion engine may also be suited to this object.
One concept for improving the fuel consumption of a vehicle, for example, is to shut the internal combustion engine off—instead of allowing it to continue to idle—when there is no instantaneous power demand. In practice the internal combustion engine may be switched off at least when the vehicle is stationary. One application of this is in the stop-go traffic such as occurs, for example, in the traffic congestion on interstate and main highways. In urban driving, stop-go traffic due to the existence of uncoordinated traffic light systems is now even the rule rather than the exception. Barrier-type rail crossings and the like represent other possible applications.
A problem with concepts, which in the absence of demand shut off the internal combustion engine in order to improve the fuel consumption, is the need to restart the internal combustion engine. Restarting presents problems among other things because in uncontrolled shutting-off of the internal combustion engine, the crankshaft and the camshaft come to rest in any unknown position. Consequently the position of the pistons in the individual cylinders of the internal combustion engine is likewise unknown and left to chance. This information, however, is essential for uncomplicated restarting in the shortest possible time with the maximum possible fuel-saving.
In an internal combustion engine, which is equipped with electronically controlled ignition and/or electronically controlled fuel injection, markers arranged on the crankshaft and/or the camshaft deliver crankshaft angular position signals to sensors connected to the engine management system for controlling the ignition timing and the injection timing. In order to generate these signals, however, it is first necessary to set the crankshaft into rotation. Right at the beginning of a starting sequence the correct injection and ignition timing are generally unclear, so that a run-in phase is necessary for synchronization of the crankshaft position on the one hand and the engine operating parameters on the other.
Knowledge of the position of the individual cylinders, that is to say knowledge of the position of the individual pistons of an internal combustion engine is necessary, in order that the injection of the fuel and the initiation of the ignition of the fuel-air mixture in the individual cylinders can be performed accurately, that is to say at defined crankshaft angles, in order to thus ensure an optimum combustion with the lowest possible fuel consumption and lowest possible emissions. Furthermore, accurate injection and ignition are necessary in order to prevent self-ignition of fractions of the mixture—so-called knocking—and to ensure the smoothest, that is to say the most uniform possible running of the internal combustion engine, which is distinguished by minimum rotational oscillations of the crankshaft and hence by minimum rotational speed fluctuations. The task of controlling the injection and ignition is generally undertaken by an engine management system.
In the state of the art the position of the individual cylinders of an internal combustion engine is determined by a camshaft sensor and a crankshaft sensor, also referred to as a crank angle sensor.
The fixed crankshaft sensor arranged on the internal combustion engine here reads off signals from a ring or toothed ring, which rotates with the crankshaft and which may be provided, for example, on the flywheel. The signal generated by the crankshaft sensor is needed by the engine management system in order to calculate the rotational speed and the angular position of the crankshaft. The engine management system needs these data in order to calculate the ignition setting, the fuel injection and the fuel quantity under all operating conditions of the internal combustion engine, knowledge of the rotational speed and angular position of crankshaft being the most important items of information generated by a crankshaft sensor.
Although the rotational speed and angular position can in principle also be determined by a camshaft sensor, the rotational speed should be determined as precisely as possible, in order to ensure correct, optimum running of the internal combustion engine, for which reason the state of the art still relies on the crankshaft sensor for this purpose, since the crankshaft rotates at twice the rotational speed of the camshaft and thereby delivers a signal with a significantly higher resolution. The crankshaft sensor is also capable of producing a higher resolution because the flywheel arranged on the crankshaft can accommodate a large number of teeth or other signal generators by virtue of its relatively large diameter.
Moreover, the piston position can be determined that much more accurately by evaluating a crankshaft signal than by a camshaft signal, since the camshaft, for drive purposes, is connected to the crankshaft by way of a relatively soft drive (generally a belt or chain drive). This shows that the camshaft may not synchronously follow the movements of the crankshaft and this results in deviations of the camshaft signal from the crankshaft signal.
The camshaft sensor is needed in order to be able to determine whether the cylinder and the piston is in the combustion cycle—compression and expansion—or in the charge cycle—exhaust and induction. The crankshaft sensor only determines the position of the piston in a crank angle window of 360°. On the basis of the information from the crankshaft sensor it is possible to determine, for example, whether the piston is at top dead center (TDC) or bottom dead center (BDC). Since in a four-stroke internal combustion engine an operating cycle consisting of compression, expansion, exhaust and induction covers a crankshaft angle (CA) of 720°, however, it is essential to know whether a piston at top dead center (TDC) is at the so-called ignition TDC (ITDC) or at the charge cycle (overlap) top dead center (OTDC). This information is supplied by the camshaft sensor, so that the piston position can be clearly determined through the interaction of the camshaft sensor and the crankshaft sensor.
In practice, the position of just one individual cylinder of the internal combustion engine is usually determined by said sensors, thereby establishing the position of the other cylinders. Knowing the position of an individual cylinder, the engine management system is able to calculate the ignition timing and the injection timing for this one cylinder. With the information on the firing order of the internal combustion engine filed in the engine management system it is then possible to obtain the ignition timings and the injection timings of the other cylinders.
A distinction must be made here between the terms injection angle and ignition angle, which follow the position of the crankshaft, and the terms ignition timing and injection timing. An injection angle might be 15° CA BTDC, whereas the injection timing must be understood to mean that the engine management system, knowing the position of the piston and the rotational speed, calculates the time at which injection occurs.
The principle of the method, which uses the two sensors, that is to say the camshaft sensor and the crankshaft sensor, to determine the cylinder position, assumes that the internal combustion engine is in operation and the camshaft and the crankshaft are rotating fast enough to enable the sensors to deliver a signal to the engine management system.
In the state of the art various concepts are proposed in order to facilitate restarting.
The German published patent application DE 42 30 616, for example, proposes to store the angular position of the crankshaft registered at the time of shutting off, and to use this for restarting, so that the suitable ignition timings and injection timings are immediately available. Should this stored information on the last position of the cylinders be no longer available when restarting, because it has been lost when the battery was removed and there was no power supply to the engine management system, for example, the state of the art allows for injection and ignition at any point when starting, the internal combustion engine, with the aid of the engine management system, adjusting to the required operating point within a couple of operating cycles. Even with a power supply, however, it has been shown in practice that the angular position of the stationary crankshaft can only be detected very imprecisely with the conventional sensors. In this context there are problems stemming from the fact that the crankshaft, at the end of the rundown sequence can also turn backwards, that is to say counter to its actual running direction, since the compressed gases in individual cylinders endeavor to expand.
Other attempts at a solution prefer methods for controlled shut-off and starting of the internal combustion engine. The controlled shut-off entails deliberately running to quite specific crank angle positions—so-called preferred positions—when shutting off the internal combustion engine. In this case the final position of the crankshaft is no longer left to chance and registered more or less accurately, crank angle positions advantageous for restarting instead being purposely adopted.
A further disadvantage of the proposed strategy, in which the internal combustion engine is shut off in the absence of any demand, in order to improve the fuel consumption, is the fact that the stop-go operation increases the demands on the starting device. For one thing the number of start sequences increases if the internal combustion engine is shut off more frequently, which calls for a correspondingly robust starting device adapted to the increased demands. For another, the starting sequence, which can take up to one second, has an adverse effect on running dynamics, and the starting noises affect the level of comfort.
In a conventional internal combustion engine having a conventional starting device, for example a starter or similar unit capable of forcing the crankshaft to rotate, such as an electric motor, for example, the internal combustion engine is started or restarted by activating the starting device and setting the crankshaft into rotation. In so doing the starting device is used to forcibly drive the crankshaft until the engine management system is synchronized and the internal combustion engine is capable of maintaining the rotation of the crankshaft without the starting device, by fuel injection and ignition of the fuel-air mixture.
Throughout the entire synchronization and beyond, until the idling speed of approximately 700 rpm is attained by virtue of the combustion processes in the individual cylinders, the starting device remains activated. The time-consuming synchronization, in particular, is responsible for the long starting times in conventional methods for starting an internal combustion engine.
In order to be able to operate an internal combustion engine in a manner consistent with the demand, especially with a view to the increasing stop-go traffic, that is to say to be able to shut it off in the absence of demand, it is therefore necessary to simplify the restarting, that is to say to make it faster and more fuel-saving. In the state of the art various concepts are proposed for achieving this aim.
The German published patent application DE 198 08 472 A1 describes a method for starting a direct-injection internal combustion engine, in which in the preliminary stages of ignition the crankshaft, in a first step of the method, is slowly turned by a drive into a position in which the piston of a cylinder is situated at top dead center (TDC). A subsequently initiated first ignition command causes the crankshaft to experience a small further rotational movement, initiating the expansion stroke. During the ensuing expansion phase fuel is injected into at least one cylinder and the fuel-air-mixture present in the cylinder is ignited, triggering or initiating the actual starting sequence.
The object of DE 198 08 472 A1 is to set forth a method of engine starting which manages with a substantially smaller current. The reasoning behind this is that starting an internal combustion engine requires substantially larger currents than normal running or normal operation of the internal combustion engine, for which reason the design of a vehicle battery, as a compromise solution, must take account of two load cases.
The initial rotation of the crankshaft and positioning of a piston at top dead center (TDC) is intended to bring the piston of a cylinder into a stable position, in which the piston is not driven forwards by an expanding cylinder charge and in which no reverse rotation occurs owing to the reversal of an incomplete and uncompleted compression.
In a departure from this DE 198 08 472 A1 proposes an alternative method, in which the piston of a cylinder is brought into a position just after the TDC-position through a driven rotation of the crankshaft.
The rotational movement of the crankshaft generated by a drive at the start of the method is not comparable with the forcible rotation of the crankshaft initiated by a starting device, which is already an integral part of the actual starting sequence, whereas the positioning of the piston according to DE 198 08 472 A1 is to be regarded only as preparation for starting.
Given a suitable position of the stationary crankshaft, in which a piston is already at top dead center (TDC) or just after top dead center (TDC), restarting from stationary is then even possible without the starter. In the process, fuel is injected directly into the combustion chamber of the corresponding cylinder of the stationary internal combustion engine and ignited by a spark plug, so that the firing of the air-fuel mixture sets the piston in motion, causing the crankshaft to rotate.
The German published patent application DE 100 24 438 A1 describes a similar method for starting an internal combustion engine. In this method also, in a so-called positioning phase, an electrical machine brings the crankshaft into a start position prior to each starting sequence, this start position being characterized in that the piston of at least one cylinder is brought into a position before top dead center (TDC).
In the ensuing starting phase an initial combustion with reduced compression and reduced volumetric efficiency is initiated in at least one cylinder, which is in the compression phase, this combustion being intended to support the torque of the electrical machine acting on the crankshaft in the starting phase.
A disadvantage to the two methods described in the state of the art is that prior to each starting sequence a positioning phase is necessary, in which the piston of at least one cylinder is brought into a position advantageous or necessary for the actual starting sequence. This positioning takes additional time and prolongs the starting sequence considerably. As already stated above, a longer starting time has a detrimental effect on the running dynamics and the level of comfort.
For this reason DE 198 08 472 A1 even proposes to initiate the positioning, that is to say the turning, of the crankshaft by a central locking remote control, in order thereby to avoid the time lost by the positioning necessary before each starting sequence. The principle underlying this variant makes it suitable only for restarting the internal combustion engine after leaving the vehicle and not for the urban stop-go traffic, in which a number of restarts are called for within a short time span.
In this context, the present description sets forth a method for starting an internal combustion engine according to the preamble of claim 1, which overcomes the known advantages inherent in the state of the art, the particular intention being to shorten the starting times.
This is achieved by a method for starting a direct-injection internal combustion engine equipped with an engine management system and having n cylinders, in which n pistons oscillate between a top dead center (TDC) and a bottom dead center (BDC), and a crankshaft, wherein proceeding from a stop position of the crankshaft known to the engine management system, a starting device, which sets the crankshaft in rotation, is activated in order to start the internal combustion engine, and whilst the crankshaft is still stationary fuel is injected into at least one cylinder, which is in the compression phase, and the fuel-air-mixture present in this one cylinder is ignited, thereby supporting the starting device.
In contrast to the methods known in the state of the art, the method according to the description dispenses with a positioning phase. The initiation of the combustion processes supporting the starting device is undertaken in the form of a fuel injection into at least one cylinder whilst the crankshaft is still stationary.
That is to say the injection occurs even before activation of the starting device or at the latest simultaneously with activation of the starting device. Proceeding from a known stop position of the crankshaft, fuel is injected into the cylinder, which is in the compression phase on the way to top dead center (TDC), it being also possible to inject fuel into more than one cylinder if there is more than one cylinder in the compression phase. Advantageously this is also done because the combustion gases expanding in the combustion chamber of each cylinder contribute proportionately to the drive torque exerted on the crankshaft by the gas forces and because the starting time is reduced as the number of cylinders increases.
The absence of the positioning phase shortens the starting sequence considerably, the absence of the positioning also saving the energy required for the positioning, which improves the overall efficiency of the internal combustion engine. According to the description the combustion processes initiated in the cylinders and the starting device mutually support one another, the two torques, that is to say the torque exerted on the crankshaft by the starting device on the one hand, and the torque exerted on the crankshaft by the gas forces as a result of the combustion processes on the other, are superimposed on or added to one another to form a common drive torque.
The method proposed according to the description permits rapid and in particular fuel-saving restarting, thereby also reducing the quantity of pollutants generated in the starting procedure. In a favorable scenario the support for the starting sequence through the application of an external torque by a starting device—for example a starter or a starter-generator may be terminated directly upon or shortly after reaching top dead center (TDC) for the first time. In a four-cylinder-in-line engine this generally corresponds approximately to one quarter-revolution of the crankshaft. Shortening the starting time improves the running dynamics and in particular the level of comfort due to the lower noise emissions. Since the position of the crankshaft is known when restarting commences, the correct injection timing and ignition timing are clear, so that only a very short, if any, run-in phase is required for synchronization of the engine operating parameters. The various possible ways of determining the crankshaft position on commencement of the starting process will be explored below in the connection with the preferred embodiments of the method.
The method according to the description therefore overcomes the known disadvantages inherent in the state of the art, a shortening of the starting times, in particular, being achieved.
Further advantageous embodiments of the method will be discussed in connection with the detailed description.